Heat 1 tablespoon oil in a large Dutch oven over medium. Add potatoes, carrots, onion, and celery; cook, stirring occasionally, until onion softens, about 8 minutes. Add tomato paste; cook, stirring constantly, 1 minute. Stir in beer and 1 tablespoon Worcestershire sauce. Bring to a gentle boil over medium-high, and cook for 4 minutes. Add stock, and bring to a boil over high to cook off the alcohol. Reduce heat to low; simmer, uncovered, 20 minutes, stirring once or twice.

While stock mixture comes to a simmer, prepare the meatballs: Stir together parsley, panko, half-and-half, garlic, egg, 1 teaspoon mustard, 1 teaspoon salt, and ½ teaspoon pepper until thoroughly combined. Add lamb, using your hands to combine. Shape lamb mixture into 20 (about 1 ½-inch, 1 ⅒-oz. each) meatballs (about 1 ½ tablespoons each); transfer to a large plate.

Heat remaining 2 tablespoons oil in a large skillet over medium. Cook meatballs, turning occasionally, until browned on 2 to 3 sides, about 10 minutes. Transfer meatballs to a large plate.

Add meatballs and any accumulated juices from plate, thyme, rosemary, and remaining 1 ½ teaspoons salt and ½ teaspoon pepper to simmering stock mixture. Return to a simmer, uncovered, over medium-high. Reduce heat to low to maintain a gentle simmer; cook, stirring occasionally, until vegetables are tender and a thermometer inserted into center of meatballs registers 160°F, about 20 minutes, stirring in roux during last 5 minutes of cooking to thicken stew.

While meatballs simmer, prepare the roux: Melt butter in a medium skillet over medium, stirring occasionally. Whisk in flour, and stir until smooth. Cook over medium, whisking constantly, until roux is lightly browned, 3 to 4 minutes. Remove from heat, and set aside until ready to use.

Remove thickened stew from heat; stir in remaining 1 tablespoon Worcestershire and 2 teaspoons mustard. Divide stew among bowls.